Model Boat Mayhem

Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: Microyacht on June 21, 2010, 01:25:36 PM

Title: Plastic Dome blowing
Post by: Microyacht on June 21, 2010, 01:25:36 PM
I want to make a 5" diameter shallow dome using transparent Plasticard. This stuff is as clear as acrylic (Perspex). The material I'm using is 1mm thick. I'm making a Airbox with a removable plate that has a 5" diameter hole in it which determines the diameter of the dome. The dishing required is very slight, 1" to 2" max. I gather that Plasticard softens at 80C. I have an airline which is fully variable from 0 to 120psi (up to 8CFM! LOL)

Question: Will it be possible to immerse in a container of hot water , slowly bringing up the temperature to 80C or more? Will it blow to shape at 80 or will it need to be 100 or more? Seems easier to soften using near boiling water than placing in the oven?

Advice please
Title: Re: Plastic Dome blowing
Post by: Circlip on June 21, 2010, 02:46:04 PM
You have a bit of suck it and see with this one Steve. Commercially, some Motorcycle screens were made by heating acrylic or Polycarbonate and blowing them up through a template with a height limit marker and held at pressure till they set. The thickness of the material worked to its advantage and even at 1mm thickness you don't have much to play with with respect to be able to support itself in cooling.

  Normal method for toy canopies is "Drape" or "Stretch" molding over a former or suck it into a dish mold by "Vacuum" forming.

  Regards  Ian.
Title: Re: Plastic Dome blowing
Post by: Guy Bagley on June 21, 2010, 04:38:45 PM
vacuum forming or blowing ( blow moulding) will reduce the wall thickness remember..... dont push it too far as you may find in punctures and splits....my first trial would be to slowly bring the temp in in say water until near boiling rather than in the oven, - its too easy to damage the surface in ' oven' heat, not so easy to spoil it in boiling water......worth a try anyhow....
Title: Re: Plastic Dome blowing
Post by: Microyacht on June 21, 2010, 05:02:35 PM
 . . . Thank you both for your suggestions. I like the idea of doing it in water. Much easier to control the temperature. I don't have a vacuum pump and in the past have tried modifying my compressor to use the Air intake as a vacuum. The compressor didn't like it and cut-out!

I've looked at the Dome to be copied and I think that my 1/8 scale is probably nearer only 1" high ( x 5" diameter). I think it will be OK  with 1mm think Plasticard. Got to be OK cos my supplier only has "clear" in 1mm.
Title: Re: Plastic Dome blowing
Post by: Circlip on June 21, 2010, 05:13:22 PM
Notice I put the term Vacuum in inverted commas' Steve. it's a bit of a misnomer as it's air pressure that "Blows" the shape into the form or over mold blocks. As you're using Styrene material, don't forget you can make use of a hot air gun rather than bake it with the Yorkshires. Even baat Yorkshires, food tastes c**p when basked in Styrene fumes.   O0

  Never heard of "Vacuum Bagging" for skinning toy Aircraft wings? You use the suction side of a household Vac.

   Regards  Ian
Title: Re: Plastic Dome blowing
Post by: malcolm.howard2 on June 22, 2010, 10:00:24 PM
Try E.M.A. model supplies limited  120mm is 6.71  many sizes to chose from Malcolm.Howard
Title: Re: Plastic Dome blowing
Post by: Microyacht on June 23, 2010, 09:57:52 AM
Quote
Notice I put the term Vacuum in inverted commas' Steve. it's a bit of a misnomer as it's air pressure that "Blows" the shape into the form or over mold blocks. As you're using Styrene material, don't forget you can make use of a hot air gun rather than bake it with the Yorkshires. Even baat Yorkshires, food tastes c**p when basked in Styrene fumes.   

  Never heard of "Vacuum Bagging" for skinning toy Aircraft wings? You use the suction side of a household Vac.

   Regards  Ian

I've made parts for full-size sailing yachts and power cruisers vacuum bagging and infusion but no longer have access to vacuuming gear. Blowing is the best option for me and I think I can make it work.

Looking at the dome again, I would say I only need to inflate it by 1". It really is shallow but really looks the part as a sky observation dome. You can just make it out in the pictures of the full-size houseboat: http://www.berkeley-engineering.com/CapeCodder.html (http://www.berkeley-engineering.com/CapeCodder.html) I'll post a picture of my 1/8 scale model later.
Title: Re: Plastic Dome blowing
Post by: Microyacht on June 23, 2010, 10:01:06 AM
Quote
Try E.M.A. model supplies limited  120mm is 6.71  many sizes to chose from Malcolm.Howard

Thanks for the info. Price would be great . . BUT I think they only make them in grey ABS. I've emailed them. Would be an alternative if all my work goes wrong!

Update: Just found the "Clear" ones, like: http://www.ema-models.co.uk/products/03143-h-dome-clear-acrylic.html (http://www.ema-models.co.uk/products/03143-h-dome-clear-acrylic.html)
Title: Re: Plastic Dome blowing
Post by: Circlip on June 23, 2010, 12:51:47 PM
I wasn't doubting your abilities Steve, the problem with inflating the hot styrene to the correct height is one of micro control of the pressure and the heat. A bit different on a production basis using a "Tougher" material.

 Regards  Ian
Title: Re: Plastic Dome blowing
Post by: Microyacht on June 23, 2010, 06:22:56 PM
I might blow it up-side-down. Easier to see when the plastic starts to sag. Just need to get the temperature and pressure right. Hey, this stuff is only just over 1 a 12" X 7" sheet! Scheduled for Friday/Saturday . . . Fingers crossed!
Title: Re: Plastic Dome blowing
Post by: Microyacht on June 24, 2010, 09:53:35 PM
ALL my boats are sailing yachts apart from my Houseboat. All undercoat finished. Topcoat next. All the gables go blue as well. The 5" low profile dome (skylight) goes on the roof of the smaller dormer on the left hand side in the picture:

(http://www.withamweather.co.uk/misersdream/images/rc-cc-painting-6.jpg)